The present invention generally relates to coatings for niobium (Nb)-silicide (Si) based solid solution alloys and Nb—Si based composite compositions. In particular, the invention relates to coatings for Nb—Si based solid solution alloy and Nb—Si based composite compositions having properties that permit the Nb—Si based solid solution alloy and Nb—Si based composite compositions to find applications in turbine components.
Turbines (and their components) such as, but not limited to, aeronautical turbines, land-based, turbines, marine-based turbines, and the like, have typically been formed from superalloys, often based on Ni. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. For example, turbine components, such as an airfoil, in modern jet engines can reach temperatures as high as about 1050° C., which can be as high as approximately 85% of the melting temperatures (Tm) of many Ni-based superalloys.
However, efforts have been made to develop alternative turbine component materials. These alternate materials include Nb-based refractory metal intermetallic composites (hereinafter “RMIC”s). Most RMICs have melting temperatures of about 1700° C. If RMICs can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service limit of Ni-based superalloys.
Examples of such RMIC's include various Nb—Si based in-situ composites. Niobium-silicon in-situ composites possess a useful range of mechanical properties, such as low-temperature toughness as well as reasonable high-temperature strength and creep resistance. However, an impediment to the development of the Nb—Si in-situ composites for high-temperature applications in aircraft engines is that their environmental resistance, e.g., oxidation behavior, is not always completely satisfactory. While further alloying with Ti, Al, Hf, Cr, and other elements has produced material systems with improved environmental resistance, further improvement in oxidation behavior is desirable for application in hot-section engine components such as airfoils, rotors, nozzles, shrouds, and exhaust components. Coating systems under development may provide environmental benefits; however, a suitable coating system has not yet been developed to meet the demanding requirements imposed by an engine environment.